Sean Neeson is standing down from the Assembly at the next election.
The former Alliance leader, 64, wants to devote more time to his family and other interests like maritime heritage.
He said: "It has been an honour and a privilege to serve the people of East Antrim at the Assembly.
"It has been very rewarding and at time extremely challenging trying to build peace in Northern Ireland.
"I want to pay tribute to all those who, during the darkest days of the Troubles, worked hard from the centre ground to bring this society forward towards a peaceful and shared future."
He is a member of the UK National Historic Ships Committee and of the Nomadic Trust.
Mr Neeson was first elected to the Assembly in 1982 and became Deputy Speaker in that Assembly. The next election is expected in May.
Mr Neeson served as Alliance leader from 1998 to 2001.
"Things like maritime heritage are very important to me and there are books I want to write," he said.
"Recently, I became a grandfather and I want to spend more time with the family."